The present invention generally relates to a matrix type liquid crystal display unit and more particularly, to a drive circuit for use in matrix type liquid crystal display unit, in which a switching transistor for addressing is provided at each of picture elements disposed in a matrix type display pattern.
Conventionally, as a matrix type liquid crystal display unit having non-linear elements used for performing display drive of liquid crystal, a TFT active matrix type liquid crystal display unit is known in which thin film transistors (referred to as "TFTs", hereinbelow) for addressing are incorporated, in a shape of a matrix, into a liquid crystal display panel such that display of high contrast equivalent to that of static drive can be obtained even in case where drive having a small duty ratio, i.e. multiplex drive of multiple lines is performed. For example, a known TFT active matrix type liquid crystal display unit has a circuit configuration of FIG. 1 and wave forms of signals of FIG. 2. The known TFT active matrix type liquid crystal display unit includes a liquid crystal display panel 11, a row electrode driver 12, a gate signal control unit 13, a column electrode driver 14 and a data signal control unit 15. In liquid crystal display panel 11, (as shown in the oval window) a TFT 11c is connected to a point of intersection between a row electrode 11a and a column electrode 11b. Reference numeral 11d denotes the capacitance of a liquid crystal layer. The above description is only an example of one of many display sites on the liquid crystal display panel. The row electrode driver 12 is mainly comprises a shift register which sequentially shifts a scanning pulse in response to a clock .phi.1 from the gate signal control unit 13 so as to output the shifted scanning pulse to each row electrode. Assuming that character T denotes a total scanning time period for scanning all the row electrodes and character N denotes the number of the row electrodes to be scanned, a scanning time period H for scanning each the row electrode, as represented by 11a, is expressed by the following equation. EQU H=T/N.
A voltage pulse having a pulse width equal to the scanning time period H is sequentially applied to each row electrode so as to turn on the TFTs one line by one line. The column electrode driver 14 is either the type in which data are directly sampled and held on the display panel 11, (referred to as "panel sample-and-hold drive type"), or the type in which the column electrodes have a function of sampling and holding data, (referred to as "driver sample-and-hold drive type".
As shown in FIG. 3, the column electrode driver of the panel sample-and-hold drive type is constituted by a shift register 31, sampling switches 32, etc. The column electrode driver samples synchronously with a clock .phi.2 at a timing corresponding to each column data transmitted in series from the data signal control unit 15 and outputs the sampled data to the column electrodes sequentially so as to write the outputted data on the liquid crystal layer through the TFTs. In the panel sample-and-hold drive type, the sampling of the data and the writing of the data on the liquid crystal layer through the TFTs are performed during the identical horizontal scanning time period.
The driver sample-and-hold drive type is described with reference to FIGS. 4 and 5. In the driver sample-and-hold drive type, the column electrode driver is constituted by a shift register 41, sampling switches 42, etc. The sampling switches 42 are turned on synchronously with output of the shift register 41 such that electric charges corresponding to the data signals are stored at capacitors as represented by 43, respectively. Subsequently, a discharge pulse signal disposed at an initial half of a horizontal blanking time period is applied to a line Cl so as to discharge remaining electric charge such that a base condition is formulated. Then, when a transfer pulse signal disposed at a last half of the horizontal blanking time period is applied to a line Cg, the electric charges stored at the capacitors are transferred to transistors as represented by 44, to be outputted. In the driver sample-and-hold drive type, the data are written on the liquid crystal layer during a time interval of the scanning time period H after sampling of the data.
In the case where the row electrodes are led from the liquid crystal display panel to the row electrode driver, there is one method shown in FIG. 1 in which all the row electrodes are led from one side of the liquid crystal display panel to the row electrode driver, or there is another method in which the row electrodes are alternately led from opposite sides of the liquid crystal display panel to the row electrode driver due to mounting conditions. In the case where the row electrodes are led from the opposite sides of the liquid crystal display panel to the row electrode driver, the signals are required to be alternately and sequentially applied to the row electrodes disposed at one side of the liquid crystal display panel and the row electrodes disposed at the other side of the liquid crystal display panel. Thus, if the row electrode driver is disposed at one side of the liquid crystal display panel, such inconveniences take place that connections for connecting the liquid crystal display panel and the row electrode driver are required to be extended longer and the wires intersect with each other. This requires that the area required for wiring be large and that the wiring should be performed by using through-holes, thereby posing problems to miniaturization and reliability of the liquid crystal display panel. Furthermore, in the case where two row electrode drivers are, respectively, disposed at the opposite sides of the liquid crystal display panel, one of the row electrode drivers delivers output signals to the cells having odd numbers counted in the shift register, while the other one of the row electrode drivers delivers output signals to the cells having even numbers counted in the shift register. Thus, each of the row electrode drivers uses only a half the parts on the shift register, thereby resulting in disadvantages in miniaturization and power consumption of the liquid crystal display panel. Meanwhile, in this case, since a start pulse signal and a clock signal are required to be applied to each of the shift registers disposed at the opposite sides of the liquid crystal display panel so as to actuate each of the shift registers, the number of input signals becomes large undesirably.
Moreover, in the above described drive types, supposing that character R.sub.ON designates the resistance of the transistor at the time of turning on and character C.sub.LC designates the capacitance of the liquid crystal layer, a time constant T.sub.ON for charging the display picture element electrode is given by the following equation. EQU T.sub.ON =R.sub.ON .times.C.sub.LC.
It is desirable that the time constant T.sub.ON is set far smaller than the scanning time period H such that the display picture element electrodes are sufficiently charged when the electric potential of the display picture element electrodes becomes equal to electric potential of a wave form of data signals. Unless the time constant T.sub.ON is far smaller than the scanning time period H, the TFTs will be turned off before the liquid crystal layer is charged to a predetermined electric potential through the TFTs by using a voltage applied to the column electrodes. By turning off early, the TFTs cause aggravation of display characteristics. In addition, in such a state, the voltage applied to the liquid crystal layer varies according to values of the time constant T.sub.ON. Therefore, if there is a variability in values of the resistance R.sub.ON and the capacitance C.sub.LC of each of the picture elements in the liquid crystal display panel, its effect becomes apparent in the display contrast and offers a serious problem in display in which half tone is necessary, for example, television picture.